In the 1940s bacteria were thought to reproduce only asexually. Lederberg mixed two Escherichia coli K-12 strains and observed progeny with new combinations of nutritional requirements. He termed the process 'conjugation,' showing that genes move in a unidirectional manner from a donor to a recipient. Once it was realized that bacteria can exchange genes, the rapid spread of antibiotic resistance could be explained. Lederberg went on to identify the F plasmid and Hfr strains, enabling chromosome mapping in bacteria. The discoveries made it possible to perform genetics experiments in organisms that divide every 20 minutes. They form the basis of modern genetic engineering and microbiome research.

Working in Tatum’s laboratory, Lederberg crossed pairs of auxotrophic E. coli strains and recovered recombinants on double-selective media. Although the recombinant frequency was only about 1 in 10^7, the phenomenon was reproducible and could not be explained by plasmid contamination or back mutation. U-tube experiments with a filter barrier confirmed unidirectional gene flow, and the transferable element was later identified as the F plasmid. The F factor initiates chromosome transfer as a pilot DNA and, because transfer is interrupted, genes enter the recipient in a time-dependent sequence. By measuring entry times from Hfr donors, Lederberg and colleagues constructed a genetic map of the circular E. coli chromosome. The linear order derived from interrupted mating suggested genome circularity, corroborated by subsequent electron microscopy. Lederberg also co-discovered transduction, gene transfer mediated by bacteriophages, expanding the repertoire of bacterial gene movement. These insights laid the groundwork for molecular cloning, which uses bacteria as gene vectors. They also underpin concepts of antibiotic-resistance islands and pathogenicity island dissemination. Modern microbiome evolution studies and emerging infectious disease models build on the same horizontal gene-transfer mechanisms he revealed.

The oriT of K-12 Hfr strains resides within an IS3 insertion in the integrated F factor; after nicking, the 5′ end is escorted by TraI helicase into the recipient. Lederberg performed interrupted-mating assays at 10-minute intervals, determining distances of 2.5 kb between lacZ and galK and 1.1 kb between galK and aroA with high resolution. Using three-point crosses with high-input nutritional markers, he obtained data that indicated linear gene order along with circular permutation of the E. coli genome. The U-tube employed a 0.45 µm cellulose-nitrate filter, ensuring that DNA-protein complexes could not traverse passively. His replicon hypothesis stemmed from the integrative plasmid model of the F factor, presaging later concepts of ICEs and IMEs. Further work with P1 and P22 phages demonstrated that generalized transduction can transfer >10 kb DNA fragments at high frequency. These findings explain the mobility of multidrug-resistance gene clusters and inform clinical resistance tracking methodologies. Today, long-read sequencing provides a powerful tool for validating his horizontal-transfer model at whole-genome scale.